1. Field of the Invention
The present invention deals with the field of steam turbines.
A steam turbine of this type is known, for example, from EP-A1 0 965 732 or from the article by L. Busse and K. -H. Soyk “World's highest capacity steam turbosets for the lignite-fired Lippendorf power Station”, ABB Review June 1997, pp. 13-22 (cf the HP turbine in FIG. 2 of that document).
2. Brief Description of the Related Art
Shrink rings which surround the inner casing of the steam turbine, holding it together and mechanically stabilizing it under the high pressures which occur, are used in steam turbines designed for high pressure or intermediate pressure, the blade passages of which have a relatively small cone angle.
One example of a configuration of a steam turbine of this type from the prior art, as is known from the article in ABB Review cited in the introduction, is illustrated in FIG. 1. The steam turbine 10 illustrated in FIG. 1 includes a rotor 11, which is mounted in two bearings 19, 19′ such that it can rotate about an axis 23. The rotor 11 is surrounded concentrically at a distance by an inner casing 14. An annular passage 12 remains clear between the rotor 11 and the inner wall of the inner casing 14, and the blading made up of rotor blades and guide vanes (21 and 22 in FIG. 3) is accommodated in this annular passage 12, through which the steam, which enters the right-hand end of the annular passage 12 radially through a live steam inlet 18, flows (from right to left in FIG. 1). For its part, the inner casing 14 is concentrically surrounded at a distance by an outer casing 13, so as to form an annular intermediate space 16 which extends approximately parallel to the annular passage 12 and is connected to the annular passage 12 at the outlet side. In operation, the steam enters the annular passage 12 through the live steam inlet 18, flows through the annular passage 12 from right to left, performing work as it does so, is diverted at the left-hand end and then flows through the intermediate space 16 to an outlet before then leaving the steam turbine 10 for further use (for example in a subsequent intermediate-pressure stage).
The inner casing 14 is of multi-part design and usually includes an upper part and a lower part which, after assembly, are held together by externally encircling shrink rings 15. For this purpose, a plurality of shrink rings 15 are distributed in succession in the axial direction over the length of the inner casing 14 (cf also EP -A1 0 965 732). In the case of the known steam turbine 10 illustrated in FIG. 1, the additional radial distance to the annular passage 12 produced by the shrink rings 15 is utilized in order to provide a heat shield 17, which reduces the temperature difference between the inner side and outer side of the inner casing 14 in the region of the steam inlet and therefore also reduces the thermal stresses at the inner casing 14, in the region of the particularly high inlet temperatures. The shield 17 includes a cylindrically bent metal sheet, which, bearing against the outer circumference of the shrink rings 15, surrounds approximately the right-hand half of the inner casing 14 and is thereby restricted to a particularly highly stressed portion of the inner casing 14. In the left-hand portion of the inner casing 14, which is not surrounded by the shield, the shrink rings 15 project unimpeded, in the form of fins, into the intermediate space 16, where they considerably impede the flow of steam flowing within the intermediate space 16.